The death of massive stars sends gravitational waves through the universe

A new study has found that the death of massive, rotating stars could send gravitational waves throughout the universe that can be detected on Earth.

The violent death of rapidly rotating stars with 15 to 20 times the mass of the Sun is an event called a collapsar. When the star has used up all the fuel in its core through nuclear fusion, it implodes and explodes, leaving behind a black hole surrounded by a disk of leftover matter.

This material flows spirally into the black hole within a few minutes – so violently that it distorts the space around it and sends gravitational waves through the universe.

Simulations describing this process in detail are described in an article published in Astrophysical Journal Letters.

After the death of a massive, rotating star, a disk of material forms around the central black hole. As the material cools and falls into the black hole, detectable gravitational waves are created, according to new research. Image credit: Ore Gottlieb.

The researchers found that these waves should be detectable with current instruments such as the Laser Interferometer Gravitational-Wave Observatory, which made the first observations of gravitational waves in 2015.

“Currently, the only sources of gravitational waves we have discovered come from the merger of two compact objects – neutron stars or black holes,” says study leader Ore Gottlieb, a research associate at the Center for Computational Astrophysics at the Flatiron Institute in New York City, USA.

“One of the most interesting questions in this area is: what are the potential non-fusion sources that could produce gravitational waves that we can detect with current facilities? A promising answer at the moment is collapsars.”

The researchers found that collapsars can generate gravitational waves strong enough to be detected at 50 million light-years away. That's less than a tenth of the distance at which gravitational waves from merging neutron stars or black holes can be detected.

However, the result opens up the possibility of detecting collapsers using gravitational waves, considering that the closest galaxy to us is the Andromeda Galaxy, which is about 2.5 million light-years away.

Gottlieb said the result is surprising because scientists had believed that the violent collapse of a dying, massive, rotating star would be too chaotic to produce waves that could be heard in the background noise of the universe.

Gravitational waves created by mergers are amplified by the fact that the objects being merged orbit each other in close orbit.

The new simulations show that collapsars can produce a similar effect due to the spiral disk of matter surrounding them.

“I thought the signal would be much more chaotic because the disk is a continuous distribution of gas with material rotating in different orbits,” says Gottlieb. “We found that the gravitational waves from these disks are emitted coherently and are also quite strong.”

New gravitational wave detectors could discover dozens of collapsars each year. But to find them, more stellar masses and rotation profiles must be simulated.

“In principle, we would ideally simulate a million collapsars in order to be able to create a generic template, but unfortunately these are very expensive simulations,” says Gottlieb. “That's why we have to choose other strategies for the time being.”

Gottlieb's team performed the calculations on a small sample of star types. Scientists can now begin combing through historical data to see if there are any matches in the archives.

The discovery of such events would help astrophysicists better understand the extreme physics of the collapse of stars and black holes.

“These are things that we cannot otherwise detect,” says Gottlieb. “The only way for us to study these inner regions of the star around the black hole is through gravitational waves.”